This invention relates to a method for extracting heavy metals from sulphurated, or partly sulphurated, mineral concentrates containing them. More specifically, the invention concerns a method for extracting low-melting heavy metals from the respective sulphurated concentrates, and it will be understood that, even if specific reference is made hereinafter to the extraction of lead, the inventive method is equally suitable for the production of other low-melting heavy metals from respective, at least in part sulphurated, mineral concentrates, e.g. for the production of zinc.
Thus, with illustrative reference to lead, it is known that the production of primary lead is currently accomplished mainly through pyrometallurgic processes which, in accordance with traditional dual-stage techniques, involve first the oxidation and sintering of the concentrates, followed by their reduction with coal in a wind furnace, and according to more recent single-stage processes, involve direct smelting of the concentrates with oxygen, yielding impure lead (lead bullion) and sulphur dioxide.
The aforementioned pyrometallurgical methods have certain recognized drawbacks, among which, in the instance of the cited traditional processes, high material and energy consumption, and serious environment pollution problems, this latter disadvantage affecting also the more recently developed processes cited above. In fact, not completely solved in the direct smelting processes is the problem of dust and fume generation (the potential harmfulness whereof is well known, especially in the event of escape and leakage), as due also to the fact that the operating temperatures equal or exceed those of the traditional two-stage methods of reducing lead.
As an alternative to the pyrometallurgical methods, lead extraction methods by electrolysis in molten salts have also been proposed. In such electrolytical processes, the raw material used is galena (PbS) dissolved in a bath of molten chlorides. However, the electrolytic process for the production of lead from concentrates has proved to have some drawbacks both as carried out originally and as improved thereafter. First of all, a low process efficiency, as due to the sulphur released at the anode tending to dissolve in the electrolyte melt to form polysulphides. Moreover, the gangue present in the concentrates accumulates in the electrolyte bath, to create unacceptable slurries.